Michael J. Hoffmann

Correlation between microstructure, strain behavior, and acoustic emission of soft PZT ceramics

Abstract The influence of grain size and domain configuration on the microscopic and macroscopic electromechanical properties of soft lead zirconate titanate (PZT) ceramics were studied. Fine- and coarse-grained PZT ceramics with an average grain size of 1.5 and 3.5 μm were prepared using the conventional mixed oxide route and different sintering conditions (1225°C/2 h and 1300°C/10 h). The experiments were performed on PZT with different Zr/Ti-ratios in order to obtain pure tetragonal (45/55), pure rhombohedral (60/40), and morphotropic (54/46) compositions. To distinguish between the intrinsic and extrinsic effect microscopic strain measurements (Synchrotron X-ray diffraction) were compared with macroscopic measurements of the mechanical hysteresis loop. The results were correlated with acoustic emission measurements which microscopically reflected the intrinsic and extrinsic contributions to the macroscopic strain of the different crystallographic modifications.

Nanodomains in morphotropic lead zirconate titanate ceramics : On the origin of the strong piezoelectric effect

The outstanding piezoelectric properties of lead zirconate titanate (PZT) ceramics with compositions close to the morphotropic phase boundary of the quasibinary phase diagram of lead zirconate and lead titanate are still under debate. A combination of ex situ and in situ transmission electron microscopy and high resolution x-ray diffraction revealed that the extrinsic piezoelectric effect in morphotropic PZT is closely connected to the existence of nanodomains. The in situ transmission electron microscopy investigations with applied electric field show that mainly the nanodomains respond to the electric field while the microdomain structure does not change noticeably in our experiments.

Cyclic fatigue due to electric loading in ferroelectric ceramics

Abstract Mechanical fatigue of ferroelectric ceramics due to bipolar cyclic electric loading is examined. Beam specimens out of three different PZT materials were cut and precracks were initiated by Vickers indentation. The specimens were loaded by alternating electric fields varied from 0·9 to 1·0 and 1·5 times the coercive field Ec. In short intervals the crack propagation was measured. Before and after fatigue experiments electric polarisation and strain were measured as a function of the electric field. The crack growth rate decreases with increasing cycle number, and a saturation point is reached after approximately 105 cycles. A correlation between growth rates and ferroelectric strain was detected. Measured strain loops suggest that switching of ferroelectric domains undergoes a strong fatigue effect. Therefore after 106 cycles the elastic strain is not as strong a driving force for further crack extension. In addition fatigue-crack growth is strongly dependent on the material and the electric field strength.

Stabilizing the ferroelectric phase in doped hafnium oxide

The ferroelectric properties and crystal structure of doped HfO2 thin films were investigated for different thicknesses, electrode materials, and annealing conditions. Metal-ferroelectric-metal capacitors containing Gd:HfO2 showed no reduction of the polarization within the studied thickness range, in contrast to hafnia films with other dopants. A qualitative model describing the influence of basic process parameters on the crystal structure of HfO2 was proposed. The influence of different structural parameters on the field cycling behavior was examined. This revealed the wake-up effect in doped HfO2 to be dominated by interface induced effects, rather than a field induced phase transition. TaN electrodes were shown to considerably enhance the stabilization of the ferroelectric phase in HfO2 compared to TiN electrodes, yielding a Pr of up to 35 μC/cm2. This effect was attributed to the interface oxidation of the electrodes during annealing, resulting in a different density of oxygen vacancies in the Gd:Hf...

Composition dependence of the domain configuration and size in Pb(Zr1−xTix)O3 ceramics

The composition dependent variation of domain configuration and size in Pb(Zr1−xTix)O3 (PZT) has been investigated in a detailed transmission electron microscopy study in the range of 0.40⩽x⩽0.55. Single phase composition, Pb(Zr0.45Ti0.55)O3 and Pb(Zr0.60Ti0.40)O3, the former belonging to the tetragonal, the latter to the rhombohedral phase, feature small microdomain widths coupled with a pronounced bimodal domain distribution. Samples with compositions around the morphotropic phase boundary exhibit a decrease of bimodal distribution and an increase in microdomain width associated with nanodomain formation. The investigation of micro- and nanodomains, as well as the bimodal distribution of microdomains in undoped PZT ceramics, with respect to composition, is reported. We define nanodomains as “domains arranged within microdomains possessing a width of a few nanometers.” The strict alternation of the two orientation variants of microdomains is denoted as “bimodal domain distribution,” and is characterized ...

Debonding behavior between β-Si3N4 whiskers and oxynitride glasses with or without an epitaxial β-SiAlON interfacial layer

Abstract In order to gain insight on the influence of intergranular glass on the fracture toughness of silicon nitride, the debonding behavior of the interface between the prismatic faces of β-Si3N4 whiskers and oxynitride glasses was investigated in model systems based on various Si–(Al)–Y(Ln)–O–N (Ln: rare-earth) oxynitride glasses. It was found that while the interfacial debonding strength increased when an epitaxial β′-SiAlON layer grew on the β-Si3N4 whiskers, the critical angle for debonding was lowered with increasing Al and O concentrations in the SiAlON layer. Only in the absence of a SiAlON epitaxial layer, were debonding conditions altered by residual stresses imposed on the interface due to thermal–mechanical mismatch. A possible explanation for the effect of SiAlON formation and its composition on the debonding behavior is suggested by first-principles atomic cluster calculations. It is concluded that by tailoring the densification additives and hence the chemistry of the intergranular glass, it is possible to improve the fracture resistance of silicon nitride.

SiC/Si3N4 nano/micro-composite — processing, RT and HT mechanical properties

Two SiC/Si3N4 nano/micro composites were prepared from a starting mixture of crystalline α-Si3N4, amorphous SiNC, Y2O3 and/or Al2O3. The composite material for room temperature (RT) application has high strength of 1200 MPa, Weibull modulus of 19 and moderate fracture toughness of 7 MPa m1/2. The composite for high temperature (HT) application, without Al2O3 has RT strength of 710 MPa and is able to keep 60% of its RT strength up to 1300°C. The creep resistance of composite material is approx. 1 order higher compared to relative monolith up to 1400°C.

Microstructure and mechanical properties of carbon-carbon composites with multilayered pyrocarbon matrix

The effect of matrix microstructure on the mechanical properties of carbon fiber felts infiltrated by isothermal chemical vapor infiltration (CVI) has been studied by optical microscopy, scanning electron microscopy and three-point bending tests. The nonbrittle fracture behavior of the investigated composites is related to multiple crack deflections caused by the interfacial sliding between pyrocarbon layers with a varying texture degree and the delamination microcracking within the highly textured pyrocarbon layer. An increase of the flexural strength is observed by the composite having a multilayered pyrocarbon matrix.

Three-dimensional organization of rare-earth atoms at grain boundaries in silicon nitride

Used in the preparation of Si3N4 components, rare-earth elements promote the growth of needlelike grains essential to elevated toughness; evidently, La is significantly more effective than Lu. To explore this difference, we determine the three-dimensional organization of rare-earth atoms in the amorphous phase near prismatic interfaces in La- and Lu-containing Si3N4 using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and image processing. Evidence is presented for substantial atomic structure in notionally amorphous volumes. While the atomic arrangement in the amorphous phase conforms to the periodicity of the terminating crystal plane in both cases, the attachment sites are very different.

Characterization of ferroelectric domains in morphotropic potassium sodium niobate with scanning probe microscopy

Lead-free piezoceramic potassium sodium niobate in its morphotropic composition was synthesized with abnormal grain growth. Ferroelectric domain patterns were imaged with piezoresponse force microscopy. Analysis of the domain structure at the morphotropic phase boundary revealed a coexistence of tetragonal and orthorhombic polarized domains in a single grain.